Electrographic dot gain and optical density decoupling method, xerographic image reproduction, and systems, methods and software related thereto

ABSTRACT

A method is provided for decoupling the control of dot gain and optical density in electrophotographic based printing by varying the exposure level for each dot in an image.

BACKGROUND

In this specification, electrographic process means a process forconverting a digital image comprising pixels into a latent imagecomprising dots using light or other exposure means, e.g. from a lightsource arranged to act on a photoconductive surface, by striking thesurface, to form the latent image on the surface by changing the chargedistribution on the surface in the regions of the dots, applying atoner/liquid ink to the surface such that the toner/liquid ink adheresto the surface in regions of the latent image and transferring the tonerfrom the surface to a substrate to form a final image. The latent imagecorresponds to a digital image which is required to be reproduced. Someexamples of xerographic machines which use xerographic processes arelaser printers, digital printing presses, photocopiers, fax machines,plate setters, direct-to-film laser printers and scanned laser displays.

The term dot is intended to cover any shape which is produced by thelight source when forming the latent image, e.g. circles, dashes, linesetc, and could be considered to be “pixel”, and is not limited to anyparticular shape. For example in most laser printers these dots would besubstantially circular since they are formed by light from a laserstriking a photoconductive surface at a point corresponding to a pixelto be reproduced and charge distribution is affected substantiallysymmetrically outwardly from this point.

In this specification dot gain means the dot gain associated with anelectrophotographic process i.e. it is an expression of the sizedifference between the dot in the final physical image of thexerographic process (e.g. on paper) compared to the electronic, digitalcoverage in an original image being copied/printed etc. For example ifthe xerographic process is used to reproduce an original digital imagecomprising a pixel, the area covered by toner forming a dot representingthe pixel in the final physical image will be different to the areacovered by the pixel in the original electronic digital image.

Dot gain can be defined in a number of ways. For example, using theabove example, dot gain can be defined as the logarithm of the ratio ofthe actual dot area (in the final image) and the digital pixel area (inthe original image). Alternatively this dot gain can be expressed as thedifference between covered area in the final image (i.e. area covered bydots) and covered area in the original image (i.e. area covered bypixels). These two definitions are examples of ways in which dot gaincan be defined and both of these examples have the same sign(positive/negative) structure. Using these definitions, if the coveragein the original and final images is the same then the dot gain will bezero. In most printing processes the dot gain is usually non-zero andpositive. Using the above example to illustrate, the coverage of the dotin the final image is usually greater than the coverage of the pixel inthe original image which the dot represents.

The level of dot gain in an image formed using a xerographic process isdependent on, amongst other things, the way in which the light sourceacts on the surface to form the latent image. The extent to which lightfrom the light source changes the charge distribution on thephotoconductive surface affects the amount of toner or liquid ink (orother pigmenting material) which will adhere to the surface andtherefore affects the level of dot gain. As an example, a first latentdot (at the photoconductive surface) may be formed using a xerographicprocess by a light source discharging a region on a charged surface at afirst laser intensity for 0.1 seconds and a second latent dot may beformed using the xerographic process by the light source discharging aregion on a charged surface at the first laser intensity for 0.2seconds. The first and second regions may be discharged to differentextents which may cause different amounts of ink or toner to adhere tothe surface and thus to form the final image. This can affect the areacovered by the ink or toner in the final image. Therefore the way inwhich the light source acts on the surface can affect dot gain.

In this specification the light source level is used to indicate howmuch light from the light source acts on the photoconductive surface. Asdiscussed, this is related to the extent of change in chargedistribution on the surface in regions where the light strikes and thusthe amount of toner/ink which will adhere to the surface and is thuslinked to the level of dot gain. Some other examples of how to vary thelight source level received at the photoconductive surface are byoperating the light source in different modes (e.g. power modes orscanning modes) for different periods of time, by operating the lightsource in bursts, by operating the light source at differentintensity/power levels or by causing different amounts of light to actupon the surface in any other suitable way. If the light source is alaser one way of achieving a variation in the light source level is bylaser power modulation or by laser pulse width modulation. Light acts onthe surface by hitting the surface. Different amounts of light acting onthe surface will cause different amounts of ink/toner to adhere to thesurface in desired regions. Light source, in this specification cantherefore be used to refer to, for example a laser, optics associatedwith the laser and scanning means, e.g. a polygon mirror associated withthe laser, all in combination.

Optical Density (OD) is defined as the absorbance of light by a(printed) element and is defined as

${OD} = {\log_{10}( \frac{I}{IO} )}$

where IO is input light amplitude and I is output or reflected light.The OD of a print is dependent on the toner/ink thickness and on thecoverage. For a solid patch where coverage is, by definition, completethe OD is dependent only on the toner thickness.

The common situation in Xerographic print that the final toner or inkthickness on the substrate (e.g. paper) and the area covered aredetermined by the latent image formed by the light source on thephotoconductor and the interaction of the various voltage potentialsdriving the charged toner in the system. If the overall light amplitudeis reduced the horizontal dimension (orthogonal to thickness) of theprinted elements, or the coverage, will be largely reduced and thethickness of toner will be somewhat reduced. On the other hand if thevoltage potentials are changed then the toner thickness will be largelychanged and the coverage will be somewhat changed. Thus, normally, thethickness, resulting in Optical Density and the Dot Gain (DG) (which isa measure of the actual cover) are coupled and one may not change onewithout affecting the other. The color consistency of solid patchesdepends mainly on OD, while the width of graphic elements such astext/lines etc, depends mainly on the Dot Gain.

According to one aspect of the present invention, there is provided amethod for decoupling the tuning of Optical Density (OD) from the tuningof Dot Gain (DG), both on a global basis (same dot gain across thepage), and on a local basis where the dot gain is adapted to local imagecharacteristics, e.g. to protect sensitive graphical elements such assmall dots from print instabilities by locally modifying the dot gain.

Advantageously, accurate and consistent tuning of both Dot Gain andoptical density is obtained in the same image. However, as FIG. 21illustrates, the coupling of OD and DG can limit the possibility toobtain a desired Dot Gain value (e.g. zero) for a given OD value that istuned for solid-patch color consistency.

The connection between ink thickness and printed object coverage/size isillustrated in FIG. X, which displays the result of attempting to printa 3 pixel wide line. The curves 2001, 2002, 2003 are the 3 Gaussianshaped beams which, together, write the line (the height is in arbitraryunits). The dashed 2004 and dotted 2005 curves are the resulting chargedistributions on the photo-conductor (in arbitrary units) for twodifferent power levels—the dashed curve 2004 represents a higher powerlevel than the dotted curve 2005. The horizontal line 2006 depicts for acertain condition the development field indicating separation betweenforeground and background. Anything above the brown line will bebackground and anything below will be printed. The shaded box 2007represents the boundary of the common variation in development fieldneeded to compensate consumable variation. It will be noticed that thewidth for the lowest condition (represented by the arrow 2008) is muchless than the width for the highest condition (represented by the arrow2009). This shows that changing the ink thickness by changing thedevelopment voltage also changes the dot gain. The variation induced bychanging the laser power is shown by the difference in size betweenarrow 2010 (dashed line (higher power)) and arrow 2011 (dotted line(lower power)), indicating a change in dot gain. Towards the bottom ofthe curves, arrow 2013 illustrates that the two curves 2004, 2005 havedifferent depths indicating a difference in ink thickness when laserpower is changed. Thus the laser power and developer voltage (fieldinduced thereby) together affect and couple dot gain and ink thickness.

The central arrow 2012 represents the zero dot gain condition for thethree pixel-wide line. Since the arrow 2012 does not touch the dashed(higher power) curve 2005 and in this typical case the curve 2005represents the lowest allowable laser power before instability sets in,the zero dot gain condition is not accessible.

Moreover, for some system setting aimed to obtain certain OD values,small graphical elements such as small dots or narrow lines, may sufferfrom print instabilities if the dot gain is insufficient. Therefore itcan be desirable to increase the dot gain for such elements (“protect”them), without modifying the overall OD which is already tuned forsolid-patches.

According to another aspect of the present invention, there is provideda method to control the dot gain separately from optical density, byirradiating edge (or close to edge) dots differently from internal dots,since the dot gain is defined at the edges of the printed elements, anddoes not depend on internal dots. The provided method can bring the dotgain to a desired nominal value, in particular, zero dot gain.

According to another aspect of the present invention, there is provideda method to improve print stability, by locally adapting the dot gainaccording to local characteristics of the latent image, so that smallgraphical elements such as small halftone dots are printed in a stablefashion, i.e. always appear on the final print and preferably with aconstant size.

According to another aspect of the invention, there is provided a dotgain compensation method for taking into account dot gain in axerographic process which comprises converting a digital imagecomprising pixels into a latent image comprising dots using light from acontrollable light source arranged to strike a photoconductive surfaceand change charge distribution on the surface to form the dots makingthe latent image on the surface, the digital and latent images eachhaving an edge and comprising an edge pixel or edge dot respectively,which is at or near the edge, and a non-edge pixel or non-edge dotrespectively, which is not at or near the edge, wherein the methodcomprises the step of identifying whether or not a dot to be formed isan edge dot and using a different light source level incident at thephotoconductive surface when forming the edge dot compared to whenforming the non-edge dot such that charge distribution is changed to adifferent extent when forming the edge dot compared to when forming thenon-edge dot.

Preferably each pixel of the digital image has an associated instructionindicating a default light source level which should be used whenforming its corresponding dot in the latent image, the method comprisingforming the edge dot using a light source level different to the defaultlight source level.

Preferably the light source acts differently by (i) operating for adifferent period of time, (ii) operating in different bursts, (iii)operating at a different intensity, (iv) scanning light across thesurface at a different rate, or (v) causing a different amount of lightto strike the surface when forming a dot in any other suitable way, or(vi) any combination of (i) to (v).

The edge dot identifying step may comprise the step of comparing aselected pixel and its neighbouring pixels to templates known to beindicative of an edge pixel to determine whether or not the selectedpixel is an edge pixel.

Alternatively the pixel may have a tag identifying it as an edge pixelor as a non-edge pixel, the method comprising reading the tag todetermine whether or not the pixel is an edge pixel.

Preferably the method includes the step of calibrating the action of thelight source on the surface so that the light source forms the edge dotso as to provide a desired level of dot gain for an edge dot in aphysical image produced by the xerographic process. Preferably thedesired level of dot gain is substantially zero.

The method may comprise using a lower light source level when formingthe edge dot than when forming the non-edge dot.

The edge dot may comprise a protected edge dot and the method comprisesthe step of identifying whether or not an edge dot to be formed is aprotected edge dot and using the same light source level when formingthe protected edge dot compared to if it were a non-edge dot or using alight source level which is not reduced to the same extent compared toif it were an edge dot which is not a protected edge dot.

The protected dot identifying step may comprise the step of comparing aselected pixel and its neighbouring pixels to templates known to beindicative of a protected edge pixel to determine whether or not theselected pixel is a protected edge pixel.

The edge pixel may have a tag identifying it as a protected edge pixelor as a non-protected edge pixel, the method comprising reading the tagto determine whether or not the edge pixel is a protected edge pixel.

The method may comprise the further steps of controlling the lightsource used in the xerographic process, the process comprisingconverting the digital image comprising pixels into a physical imagecomprising corresponding dots, the method arranged to achieve a desiredlight source level when forming edge dots such that charge distributionon the photoconductive surface is changed to a desired extent andachieve a desired level of dot gain in edge dots of physical imagesproduced by the process, the light source being operable in a pluralityof modes to produce differing levels of dot gain;

-   -   an optical ratio between two xerographically produced physical        images being defined as a ratio of mean average optical        densities of each image;    -   the method comprising using the xerographic process to produce a        first physical image having a first mean average optical density        and a first attribute which influences the mean average optical        density of the image for a given level of dot gain and a second        physical image having a second average optical density and a        second attribute which influences the average optical density of        the image for a given level of dot gain, the first and second        physical images with their associated first and second        attributes being such that at a particular optical ratio between        the first and second physical images, the level of dot gain in        the second physical image will be at the desired level,        the method comprising adjusting the light source level to        produce first and second physical images until they        substantially provide the desired optical ratio between the        xerographically produced physical images, and thereby        establishing the desired light source level.

According to another aspect of the present invention there is provided acomputer program product encoded with software code which when run on aprocessor of a xerographic machine causes a processor of the machine toinstruct a light source of the machine to operate to cause a differentlight source level when forming edge dots of an image than when formingnon-edge dots of the image in order to control dot gain in thexerographic process such that a desired final line width/dot sizeresults in the image.

According to another aspect of the present invention there is provided acomputer program product encoded with software code which when run on aprocessor of a xerographic machine causes a processor of the machine tocontrol a light source of the machine to provide a desired light sourcelevel such that charge distribution on a photoconductive surface of themachine is changed to a desired extent and a desired level of dot gainin final images produced by the process is achieved, wherein the lightsource is operable in a plurality of modes to produce differing levelsof dot gain;

-   -   an optical ratio between two xerographically produced final        images is defined as a ratio of mean average optical densities        of each image;    -   the processor is arranged to use the xerographic process to        produce a first final image having a first mean average optical        density and a first attribute which influences the mean average        optical density of the image for a given level of dot gain and a        second final image having a second average optical density and a        second attribute which influences the average optical density of        the image for a given level of dot gain, the first and second        final images with their associated first and second attributes        being such that at a particular optical ratio between the first        and second final images, the level of dot gain in the second        final image will be at the desired level,        the processor further being arranged to adjust the light source        level to produce first and second final images until they        substantially provide the desired optical ratio between the        xerographically produced final images, and thereby establishing        the desired light source level.

According to a further aspect of the present invention there is provideda computer program product encoded with software code which when run ona processor of a xerographic machine is arranged to perform the stepscaused by the computer program products of the above two defined aspectsof the present invention.

According to another aspect of the present invention there is provided amethod of making a xerographic machine, such as a printer orphotocopier, comprising installing software code encoded on a computerprogram product according to any of the previously defined aspects ofthe invention on a control processor of an existing xerographic machinearranged to be able to run the software.

According to another aspect of the present invention there is provided amethod of printing an image using a xerographic printer having aphotoconductive substrate and a xerographic light source arranged toirradiate the photoconductive substrate in pixels, the amount ofxerographic light falling on a pixel being influenced by a digital imagerepresentation of an image, the digital image having for each pixel atleast one light control parameter which is used to control the amount ofxerographic light which falls onto each pixel during the formation of alatent image on the substrate, the method comprising determining whethera pixel of the digital image is an edge pixel at the edge of a featurein the image and, pursuant to that determination, altering the amount oflight that falls on the equivalent edge pixel of the latent image on thephotoconductive substrate in comparison to the amount that wouldotherwise fall on a non-edge pixel which had the same pixel lightcontrol parameter(s) associated with it.

According to another aspect of the present invention there is provided amethod of xerographically printing images comprising setting a desireddot gain for a xerographic printer or photocopier by comparing two printimages having a different ratio of number of edge pixels to total numberof pixels so as to establish what print control settings that influencethe amount of light falling on pixels of a latent xerographic image areused to achieve a desired result of said comparing, and printing imagesusing print control settings so established.

According to another aspect of the present invention there is provided axerographic image producing machine comprising:

-   -   a photoconductive substrate adapted to produce a latent image to        be produced;    -   a control processor capable of accessing a memory containing a        digital reproduction of an image to be printed;    -   a light source;        the control processor being programmed to perform an evaluation        of the digital representation to differentiate between pixels of        a latent image to be produced on the substrate that are edge        pixels at the edge of a feature in the latent image and pixels        that are more central in features of the latent image than edge        pixels and to modify an amount of light falling on pixels of the        latent image pursuant to said evaluation.

According to another aspect of the present invention there is provided amethod of forming a xerographic image from a digital image comprisingpixels and associated light level control values adapted to control theamount of light incident upon latent image pixels, on a photoconductivesubstrate, associated with the digital image pixels, the methodcomprising illuminating latent image pixels which correspond to edgepixels at the edge of a feature in the image with a lower amount oflight per unit area than is used for pixels with equivalent light levelcontrol values that are non-edge pixels.

According to another aspect of the present invention there is provided amethod of xerographic printing using a xerographic printer having aphotoconductive substrate upon which a latent image is formed from axerographic light source and a digital image to be printed having colourintensity levels associated with pixels of the digital image, the methodcomprising determining if a pixel in the digital image is an edge pixelat the edge of a feature in the digital image, and pursuant to such adetermination differentially modifying the exposure of latent imagepixels on the photoconductive substrate dependent upon whether or notthe latent image pixels correspond to edge pixels of the digital image.

According to another aspect of the present invention there is provided adigital image with edge pixels flagged as such with an “edge pixel”flag.

Further aspects of the invention are defined in the claims.

It should be appreciated that when an aspect of an invention is claimedor described as a particular category (e.g. as a method, system, datacarrier, xerographic machine etc.) then protection is also sought forthat aspect but expressed as a different category of the claim. Forexample the first aspect of the invention may also be expressed as asystem, a xerographic machine, a method etc. For example a claim to amethod may also be expressed as a xerographic machine capable ofcarrying out the method or a data carrier having software on it whichinstructs a processor to carry out the method.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which;

FIG. 1 schematically illustrates a printer according to an embodiment ofthe invention;

FIGS. 2 a and 2 b show schematically digital images to be converted bythe printer of FIG. 1;

FIG. 3 is a flow chart representing a dot gain compensation methodaccording to an embodiment of the invention;

FIGS. 4 a and 4 b show images produced by the printer of FIG. 1;

FIG. 5 a schematically illustrates a printing system, according to anembodiment of the invention, including the printer of FIG. 1;

FIG. 5 b schematically illustrates a printing system, according to afurther embodiment of the invention, including the printer of FIG. 1;

FIG. 6 is a flowchart illustrating a dot gain compensation methodaccording to a further embodiment of the invention;

FIG. 7 is a flow chart illustrating a dot gain compensation methodaccording to yet a further embodiment of the invention;

FIG. 8 is a Matlab routine suitable for carrying out the steps of themethod of FIG. 7;

FIG. 9 illustrates templates (FIGS. 9 a to 9 i), which show how theMatlab routine of FIG. 8 works;

FIG. 10 is a flowchart illustrating an embodiment of a calibrationprocess according to a further embodiment of the invention;

FIG. 11 is a flowchart illustrating a method for setting a level of alight source used in a xerographic process to a desired level accordingto a still further embodiment of the invention;

FIG. 12 schematically illustrates examples of digital images convertedduring the process of FIG. 11;

FIG. 13 schematically illustrates a xerographic system for carrying outa xerographic process including the process of FIG. 11;

FIGS. 14 and 15 schematically illustrates examples of digital imagesconverted during the process of FIG. 11;

FIG. 16 shows examples of images produced by a standard prior artxerographic process;

FIG. 17 shows examples of images produced by a xerographic process usinga dot gain compensation method according to an aspect of the invention;

FIGS. 18 a, 19 a and 20 a are examples of images produced using astandard xerographic process;

FIGS. 18 b, 19 b and 20 b are corresponding images produced using axerographic process including a calibration method according to anembodiment of the present invention; and

FIG. 21 is a schematic illustration of the attainable DG range as afunction of OD and illustrates that there are OD values that do notpermit a zero DG.

DETAILED DESCRIPTION

Referring to FIG. 1, in a first described embodiment of the invention, axerographic machine in the form of a printer comprises a photoconductor10 that generally forms the outer surface of a rotatable cylindricaldrum. It will be appreciated that the invention is applicable to anyxerographic process and in other embodiments the xerographic machine maybe a digital printing press, a photocopier, a fax machine, a platesetter, a direct-to-film laser printer, a scanned laser display or anyother xerographic machine. During the printing process the surface ofthe photoconductor 10 is uniformly charged with static electricity by,for example, a corona discharge 12. Portions of the photoconductor 10are exposed to light 14 from a light source 15. In this embodiment thelight source 15 comprises a laser in the form of an edge emitting laser.In other embodiments a plurality of such lasers may be used or differenttypes of lasers may be used or any other light source that provides asuitable light source level for use in a xerographic process may beused. The drum is rotated so that the image to be printed is formed onthe photoconductor 10. The light 14 discharges the charge on the drum inexposed areas and leaves a discharged latent image. The latent image isthen developed by applying a toner 16, such as a dry powder ink, or aliquid electroink toner (e.g. as in LEP printing) or a pigmented drypowder toner, over the surface of the photoconductor 10. The ink ortoner 16 adheres to the discharged areas of the photoconductor 10 sothat the latent image becomes visible. The toner 16 is then transferredfrom the photoconductor 10 to a sheet of paper 18, or in otherembodiments to some other suitable medium, to support the printed image.A fuser 20 may be used to fix the image to the paper 18 by applying heatand pressure, or pressure alone, to the toner 16 on the paper 18. Thedirect-to-paper transfer system shown in FIG. 1 represents only a subsetof xerographic printers. Many xerographic printers use an intermediatetransfer drum or belt to receive the toner image from the photoconductorand apply it to the print medium. Some printers have no separate fuser,and the fusing process occurs during the transfer from the intermediatetransfer drum to the paper.

The light source 15 is arranged to receive instructions regarding anoriginal digital image, which is required to be reproduced in thexerographic process. In this embodiment the light source operates byscanning light across the surface of the photoconductor 10. Theinstructions provided to the light source 15 indicate which regions ofthe photoconductor 10 should be exposed to light 14 and the amounts oflight 14 which should strike each region that is to be exposed. In thisembodiment a printer controller 22 in communication with the lightsource 15 provides these instructions.

The apparatus shown in FIG. 1 can be used to reproduce digital imagessuch as 23 a, 23 b (see FIGS. 2 a and 2 b) using a xerographic processin which a dot gain compensation method (see FIG. 3) is employed. Thedigital images 23 a, 23 b are made up of pixels and have edges 19. Eachpixel of a digital image has a value of 0 or 1 in this embodiment toindicate whether it is blank or non-blank respectively (e.g. if notblack, the pixels are in the digital image all the same shade of black).FIGS. 4 a and 4 b show final images 25 a, 25 b produced by thexerographic process resulting from conversion of the digital images 23a, 23 b respectively. The final images 25 a, 25 b comprise edge dots 26at their edges and non-edge dots 34 which are not at their edges. Thedigital image has corresponding edge pixels 21 and non-edge pixels 23.In this embodiment an edge dot 26 is defined as a dot which has at leastone neighbouring blank dot, where ‘neighbouring’ can have variousmeanings, such as 4-connectivity (i.e. connected along sides of thepixel), 8-connectivity (i.e. connected along sides or diagonals of thepixel), ‘double-layer’-connectivity (i.e. connected by at least twoconsecutive layers of black pixels) etc. When the digital image 23 a isconverted into its corresponding final image 25 a pixels with a value of1 correspond to a dot and pixels with a value of 0 correspond to no dotin the final image. This is a binary black and white printingoperation—other colours may be used in other embodiments. Also in otherembodiments where the resolution of the binary source image is not equalto the native resolution of the printing system, the source image isfirst represented in the native resolution as a multi-level(partial-exposure) image, i.e. a greyscale image. In this case, thereare many possible edge criteria that can be used, e.g., threshold theimage and use the criteria as above, threshold the local differencesbetween neighbouring pixels, etc.

In use when the light source 15 is arranged to act on the surface of thephotoconductor 10 to produce a non-blank dot, it operates at a firstlevel if the dot to be printed is a non-edge dot. In this embodimentwhen the light source 15 acts at the first light source level on thesurface of the photoconductor 10, the xerographic process produces afinal image having dots of a desired, default optical density byoperating the laser at 100% of the intensity required to produce a blackdot (i.e. a dot having a greyscale value of 255). Referring to FIGS. 3,4 a and 4 b, when producing the edge dots 26 in the final image, thelight source 15 produces corresponding edge dots in a latent imageformed on the surface of the photoconductor 10. Edge dots in the latentimage are formed by the light source 15 acting at a light source levelwhich is different to the first light source level. In this embodimentthe light source level used to produce the edge dots of the latent imageis lower than the default light source level since it is assumed thatdot gain is positive and the system is set up accordingly.

The light source level may be reduced relative to the first light sourcelevel by providing instructions to the light source 15 to produce anedge dot in the latent image using less light 14 than when producing anon-edge dot. In this embodiment this is achieved by operating the laserat 60% of the intensity required to produce a black dot (i.e. 60% of theintensity required to produce a dot having a greyscale value of 255). Inother embodiments this may be achieved by operating the laser 15 at alower intensity, operating it for a shorter period of time, operating itin bursts for a shorter period of time or increasing scan speed of thelaser across the surface of the photoconductor 10 so that it providesless light exposure to each irradiated dot for example. Step 28 shown inFIG. 3 illustrates this dot gain compensation method.

In this embodiment it is known or expected that dot gain is positive andso the light source 15 is instructed to operate at a reduced lightsource level when acting on the surface of the photoconductor 10 toproduce dots in the latent image corresponding to edge dots 26. Howeverin other embodiments if it is known or expected that dot gain isnegative then the light source may be instructed to operate at a lightsource level different to the first light source level which isincreased relative to the first light source level when producing edgedots in a latent image corresponding to edge dots in a final image.

As an example the light source level may be reduced to between 0% to 99%of the default light source level when producing edge dots. For exampleit may be reduced to about 50% intensity of the default intensity or itmay operate for less time than at the default level e.g. for a third ofthe time (but at the same intensity), or a combination of differentintensity and different time.

In other embodiments, edge dots may be printed using a first lightsource level and non-edge dots are printed using the different lightsource level.

Referring to FIG. 5 a, in some embodiments the printer controller 22 isarranged to receive instructions from a computer processor 36 of aremote computer. The computer may be a PC for example and communicationbetween the computer processor 36 and the printer controller 22 may bevia a parallel port or a USB port for example. In this embodiment thecomputer processor 36 provides instructions to the light source 15 viathe printer controller 22 on how it should act on the surface in orderto form the latent image. In other embodiments the printer controller 22itself may generate and provide these instructions. In some embodiments(as illustrated in FIG. 5 b) the instructions are provided in real timei.e. as light 14 from the light source 15 is being scanned across thesurface of the photoconductor 10. In other embodiments the instructionsmay be provided as a set of instructions for an entire print job or forsections of a print job and these instructions can be executed whendesired. In some embodiments, the instructions are stored in a memorymodule 37. In different embodiments, the memory module can be the memorymodule of a remote computer or a memory module associated with theprinter or a memory module such as a data carrier (CD, floppy discetc.).

The instructions in the form of the digital image may be a pdf, giftiff, bitmap file or in any other suitable format.

Referring to FIG. 6 in a further embodiment the dot gain compensationmethod, included as part of the xerographic process, includes at step 38the step of identifying edge dots. In this embodiment it is notnecessary to have the edge dots pre-identified. Instead the printercontroller 22 is arranged to run software 40 which enables it toidentify edge dots 26 to be printed at a light source level different tothe light source levels used to print non-edge dots. At step 38 theprinter controller 22 is able to examine an original digital image whichis to be produced by the xerographic process in digital form—the imagecomprises pixels. In some embodiments the original image is provided indigital form initially and in other embodiments the original image maynot be in digital form but may be converted to digital form before beingprocessed by the printer controller 22. For example a raster imageprocessor may be used to provide the image in a suitable form for theprinter controller 22 to process.

Each pixel making up the digital image is assessed in order to determinewhether or not it is an edge pixel 21 which will eventually correspondto an edge dot 26 in a final image produced by the xerographic process.One way of doing this is to check whether a pixel or group of pixelsmatches a pre-defined template known to correspond to a template for anedge pixel. In this embodiment the printer controller 22 carries outthis template-matching step. Examples of templates used in otherembodiments are provided below. In other embodiments different methodsmay be used in order to identify edge dots 38. In different embodimentsedge dots can be defined differently. Edge dots are whatever a softwarealgorithm defines them as. For example, in some embodiments an edgepixel can be a pixel which differs in greyscale value by greater than230 (in a 0 to 255 greyscale system) compared to its neighbouringpixels. In other embodiments an edge pixel may be a pixel having adifferent predefined colour difference with an adjacent pixel.

In some embodiments the light source may be arranged to act differentlyon the surface for edge dots and dots near the edge compared to dotswhich are not near the edge i.e. a first light source level is used toproduce dots which are not near the edge and a different, second lightsource level is used to produce dots which are at or near the edge. Forexample when the printer controller 22 is identifying any edge dots atstep 38 it may also be arranged to identify dots near the edge forsubsequent different laser exposure. Considering an image formed ofpixels where each pixel can have a value of 1 (a non-blank pixel) or 0(a blank pixel), then an edge pixel can be defined as a non-blank pixelwith at least one blank pixel next to it. In some embodiments it may beconsidered that a pixel which is diagonally adjacent another pixel isnext to the pixel and in other embodiments it may be considered thatonly pixels which are alongside other pixels are next to those pixels.In embodiments in which pixels which are near the edge of the image alsoneed to be identified, pixels which are one, two, three or more pixelsaway from the actual edge of the image may be identified as being nearto the edge. Different types of edges may be, for example, differentshapes, greyscales, colours, shades or any other suitable distinguishingfeature. For example an edge may be defined as a boundary between blueand red pixels.

In this embodiment, the pixel distance from the actual edge for which apixel is identified as being near to the edge needs to be small enoughso that the change in perceived shade or optical density will not benoticeable. As indicated above alternatively or additionally to theembodiment disclosed in FIG. 6, the tagging of edge pixels may alreadybe provided for in the original image or may be done by the printercontroller 22 or by a raster image processor or by processing of animage which has been processed by the raster image processor e.g. bytemplate-matching. When pixels are pre-tagged as edge pixels, the lightsource level to be used to print the pixels is predetermined in someembodiments and in other embodiments it is calculated on the fly duringthe printing process. In some embodiments non-edge pixels are tagged asbeing non-edge pixels instead of tagging edge pixels in order todifferentiate between edge and non-edge pixels.

In many print jobs the requirement is to reduce dot gain since an areacovered by a dot produced by a xerographic process is greater than anarea in a corresponding dot/pixel of an original image. Therefore forsome pixels, e.g. some small dots, thin lines, diagonal connections etc.it is not actually desirable to reduce dot gain to the extent providedby the light source acting in the non-default manner. Thereforereferring to FIG. 7, in some embodiments of the invention the dot gaincompensation method used in the xerographic process includes anadditional step 42 of protecting vulnerable (or unstable) pixels. Thisstep 42 may be carried out after identifying edge pixels at step 38 orbefore identifying edge pixels or at the same time as identifying edgepixels. At step 42 the printer controller 22 identifies vulnerable(unstable) pixels and is arranged to protect them by not subjectingtheir corresponding dots in the latent image to a reduced light sourcelevel during the xerographic process or at least not reducing the lightsource level to the same extent as for other edge dots which are notidentified as being vulnerable.

Vulnerable pixels are any pixels which can be identified as being pixelswhich correspond to dots in the latent image which if exposed to a lightsource level, would make the image unclear or unstable. Referring toFIG. 8 an example of software 40 in the form of a Matlab routine run bythe printer controller 22 in order to identify and protect thevulnerable pixels and identify edge pixels is shown.

Referring to FIG. 9 an illustration of the effect of the Matlab routineis provided. Matlab is a programming language which is a proprietaryproduct of the MathWorks. The Matlab routine causes the printercontroller 22 to examine each pixel of the original digital image byconsidering each pixel as a central pixel in a group of nine pixels in athree by three square of pixels. Pixels surrounding the central pixelare then examined to determine whether the central pixel is a particulartype of pixel e.g. an edge pixel. In this Matlab routine the step 42 ofidentifying and protecting vulnerable pixels occurs before the step ofidentifying other, non-vulnerable edge pixels. Each group of nine pixelsis compared to a set of templates sequentially in order shown in orderfrom FIGS. 9 a, 9 b, 9 c, 9 d, 9 e, 9 f, 9 g, 9 h to 9 i. Each templatecomprises nine pixels, each pixel being assigned a value of 1 toindicate the presence of a non-blank pixel in the original image, avalue of 0 to indicate the presence of a blank pixel in the image and avalue of neither 1 nor 0 to indicate the presence of either a blank or anon-blank pixel in the image. The set of templates shown in FIGS. 9 a to9 i are suitable for firstly checking for a solid pixel (i.e. one whichis non-blank and completely surrounded by non-blank pixels). If aselected pixel being examined is identified as a solid pixel, i.e. as anon-blank, ‘1’ pixel surrounded completely by non-blank, ‘1’ pixels, theprinter controller 22 instructs the light source 15 to act at a firstdefault level when producing a dot corresponding to the pixel. This isbecause a solid pixel is not producing an edge dot in the final image.Next, the templates shown in FIGS. 9 b and 9 c are used to identifyvulnerable pixels which are either a single pixel or parts of a thindiagonal. These pixels are edge pixels but are identified as beingvulnerable and therefore in this embodiment there is no change in laserpower from the default level when producing a dot corresponding to thecentral vulnerable pixel. In other embodiments they may induce somechanges in laser power but not to the same extent as pixels which areedge pixels that are not vulnerable. For example normal edge pixels maybe produced at 50% of default laser intensity whereas vulnerable pixelsmay be produced using 90% of default laser intensity, for example.

In this example pixels have a value of 1 or 0 in the original digitalimage. In other examples the pixels may originally have a greyscalevalue of between 0 and 1023 for example. In this case, vulnerable edgepixels, non-vulnerable edge pixels and solid pixels can be produced atthe above-mentioned percentage multiples of their individual assignedgreyscale values, e.g. a normal edge pixel having a greyscale value of100 would effectively be produced at a greyscale value of 50 (ifproduced at 50% of default laser intensity).

Once a template match is provided for a particular pixel being examined,the pixel in question is not checked against the rest of the templatesin the sequence. The templates shown in FIGS. 9 d to 9 g correspond todiagonal connections which provide edge pixels but also have at leastone neighbouring, non diagonal, non-blank pixel. These are edge pixelsbut they are not vulnerable and so dots corresponding to the centralpixel in the templates are produced using the non-default, reduced lightsource level. The templates shown in FIGS. 9 h and 9 i correspond tonon-blank pixels which are parts of horizontal or vertical connectionsand so are edge dots which are not vulnerable. These are also printedwith a reduced light power level.

In other embodiments different template matches may lead to differentlight source levels being used for e.g. different types of edge pixelsuch as those shown in FIGS. 9 d and 9 g.

It will be appreciated that instead of a reduced laser power level thelaser may be operated at the same power but for a shorter duration oftime or may be otherwise operated to act differently, e.g. the scanspeed may be changed, in order to provide less light exposure whenforming a dot required to be formed at the reduced level.

It will also be appreciated that different levels of vulnerability forpixels may be provided so that different light source levels may beassigned for producing dots of differing degrees of vulnerability.

In other embodiments vulnerable edge pixels may be pre-identified e.g.by tagging them (similarly to how edge and non-edge pixels are tagged insome embodiments).

Advantageously, in cases where dot gain is negative and the dot gaincompensation method of this invention is employed in a xerographicprocess to provide an image in which edge dots are provided at anon-default, increased level, then the final image produced by thexerographic process is clearer than if the compensation method is notemployed.

Advantageously, if dot gain is positive and the compensation method ofthe present invention is employed to reduce the light source level whenproducing edge dots in the final image, the final image is also madeclearer. In this case, for example if there is a narrow blank gapbetween non-blank regions of an image, if the compensation method werenot to be applied then it is possible that the gap may be totally orsubstantially or at least partially closed due to the effects of dotgain whereas when the compensation method of the present invention isemployed this effect is reduced or eliminated altogether.

Referring to FIG. 10 in some embodiments of the present the dot gaincompensation method further includes, or alternatively includes, a step44 of calibrating the action of the light source 15 on the surface ofthe photoconductor 10 during the xerographic process so that the lightsource 15 forms the edge dot 26 to provide a desired level of dot gain.In this way more control is provided over the level of dot gain for edgedots in a final image. It is not merely reduced or increased relative todots produced by the default light source level, but can be controlled.Following the calibration step the printer controller has informationregarding the light source level required to achieve a desired dot gainin a particular xerographic process and can use this particular lightsource level on producing edge dots 26 at step 28. Advantageously, thecalibration step allows the light source level to be calibrated for aparticular xerographic process so factors such as the amount of chargeon the photoconductor 10, the amount of charge on the toner 16, theproperties of the substrate 18 etc. are taken into account during thecalibration process. A suitable calibration process is described indetail below.

A preferred level of dot gain to be achieved is desired, and may be zeroor substantially zero so that a final image produced by a xerographicprocess is as close as possible to an original image which is intendedto be produced by the process. Alternatively it may be required toprovide a required, predictably controlled, non-zero level of dot gain.This may be useful in situations, for example, where some fonts inprinting applications are pre-designed to account for a certain level ofdot gain and the appearance of the final image will most closely matchthe intended appearance of the original image if a pre-determined levelof dot gain is present in the xerographic process producing the image.

Referring to FIG. 11, a suitable calibration process for regulating theoperation of a light source used in a xerographic process so as toproduce a desired light source level in order to achieve a desired levelof dot gain in images produced by the process is illustrated. Thisprocess is applicable, for example, at step 44 of FIG. 10. Alternativelyit may be employed in a distinct xerographic process in which edge dotsare not required to be printed at a non-default light source level.

The apparatus shown in FIG. 1 for carrying out the xerographic processis suitable to carry out this calibration process. The light source 15is operable in a selected one of a plurality of modes of operation so asto produce a plurality of different levels of dot gain in a final imageproduced by the xerographic process corresponding to each mode ofoperation. An image produced by the process will have a certain averageoptical density (equivalent to mean optical density) which can bemeasured by how dark or light a particular image is. The optical densityof an image will be dependent upon the amount of light that can passthrough the image and in a particular xerographic process it will bedependent upon the amount of toner on a substrate upon which the imageis provided, for example. The optical density can thus provide a measureof the dot gain since the amount of toner upon the substrate is linkedto the dot gain present when printing images by the xerographic process.A measure of the optical density can be provided by measuring the amountof light that can pass through the image on the substrate or bymeasuring the amount of light that is reflected from the image on thesubstrate.

An optical ratio between two images is defined for the purposes of thisspecification as a ratio of the average optical densities of each image.Images produced by the xerographic process also have attributes whichaffect the average optical density of the image for a given level of dotgain (i.e. at a particular light source level).

The calibration process 48, at step 50, comprises the step of providingfirst and second images having desired attributes to be produced suchthat, at a particular optical ratio between the first and second images,the level of dot gain in the second image would be at the desired level.

At steps 52 and 54, first and second images respectively are produced.The first image provided at step 50 has a first average optical densityand an attribute which affects the average optical density of the imagefor a given level of dot gain. The second image provided at step 50 hasa second average optical density and an attribute which affects theaverage optical density of the image for a given level of dot gain.

At step 55 the calibration process 48 comprises the step of testing thefirst and second images to check whether they substantially provide thedesired optical ratio and thus indicate the desired light source level.If they do not then at step 56, the light source level is altered andthe process 48 is recommenced at step 52 so that more images areproduced until the desired optical ratio is reached. If, or when, afterstep 55, the first and second images provide substantially the desiredoptical ratio, then the desired light source level (i.e. that whichproduces the desired dot gain) is found (step 57).

Advantageously, the light source level using the xerographic process canthen be controlled to print some or all of an image or parts of animage. The calibration process can be redone whenever any element of thexerographic process which may affect dot gain changes—for example when asubstrate upon which an image is being produced is changed or when atoner is changed or periodically to account for factors which changeover time e.g. charge density of the surface of the photoconductor. Thecalibration process can be carried out for example when there is apredefined degree of change in a factor or factors affecting dot gain.For example the calibration process can be carried out at predeterminedtime intervals or after a predetermined amount of toner is used.

In some embodiments, the attributes at step 50 which are provided areratios of the edge dot density to the total dot density for each imageto be produced, i.e. the ratio of the area covered by edge pixels in animage to the area covered by all pixels in the image. For example, if itis desired to produce a xerographic process in which the dot gain iszero, the light source level used to produce the first image at step 52is kept the same as the light source level used to produce the secondimage at step 54 is kept the same during the adjustment step 56.Referring to FIG. 12, the first image 58 is an image to be printedcomprising a repeating pattern of vertical blocks of pixels in a “2 on-2 off” configuration, i.e. two columns of pixels are non-blank and twoare blank in a repeating unit of four pixels. The second image 60comprises a 4 on -4 off repeating pattern (and the edge dots make up adifferent fraction of the overall area of printed area). At step 50these repeating patterns are provided for the first and second images toprovide a specific ratio of edge dot density to total dot density foreach image such that when the optical ratio between these images reachesone at step 56 (by adjusting the light source level until this opticalratio is reached) then a dot gain of zero will be achieved in theimages. So, initially at step 50 the attributes (i.e. the forms of therepeating patterns) are set as indicated above. At steps 52 and 54 thefirst and second images are produced. At step 55 the images are testedto determine the optical ratio between them. If this is not at thedesired level (one in this embodiment), the light source level isadjusted and more images are produced and tested. This process isrepeated until a pair of images is produced having an optical ratio ofsubstantially one. This indicates a dot gain of zero in the imagesproduced at that light source level (for reasons explained below).

This is because the average optical density of the first image 58 can beexpressed by the formula (2+2D₁)/4. The first ‘2’ in this formularepresents the fact that within each repeating unit there are twonon-blank pixels, the second ‘2’ in the formula represents the fact thatthere are two edges in each repeating unit and D₁ is a measure of thedot gain at each edge (each edge pixel will occupy an area of not only“1” unit, but also D₁, the dot gain area). The ‘4’ in the formularepresents the fact that each repeating unit is four pixels wide, and isnecessary in order to indicate the average optical density. Similarlythe average optical density of the second image 60 can be expressed as(4+2 D₁)/8 since in each repeating unit there are four non-blank pixels,two edges, a dot gain D₁ for each edge pixel which is the same as thedot gain in the first image 58 (because the same light source level isused to produce both the first and second images 58, 60) and there areeight pixels in each repeating unit. It will be appreciated thatalthough there is a dot gain D₁ associated with “middle”, non-edge,pixels it does not actually increase the area covered by the patternsince it spreads onto an adjacent pixel that is already dark.

At step 56 the light source level is adjusted to produce images whichhave an optical ratio of one, i.e. the average optical density of thefirst image 58 is the same as the average optical density of the secondimage 60. This is because when the optical ratio is one, D₁=0, i.e. thedot gain is at the desired level of zero. In this way the light sourcelevel required to produce a dot gain of zero using the xerographicprocess is determined. In some embodiments, an operator can manuallycheck whether the first image 52 is as dark as the second image 54 whichwould indicate an optical ratio of one (i.e. they can visually inspectthe printed images and use their skill and judgement to assess them). Insome embodiments the optical ratio may be required to be one or close toone within specified limits (e.g. the limits of human observance). Inother embodiments the optical ratio may be machine-determined, forexample automatically determined by a machine and, similarly, may berequired to be close to a desired amount within a predetermined limit.

The skilled person can see that whilst the digital coverage is kept thesame between the first and second images, the ratio of edge to non-edgepixels can be varied to calibrate for zero dot gain.

FIG. 13 shows a xerographic system capable of employing the calibrationprocess 48, the system comprising the apparatus of FIG. 1 represented byreference numeral 62 and a measuring device 64. The measuring device 64may be part of the printer in some embodiments or may be providedseparately. The measuring device 64 may be a scanner for measuring theoptical density of the first and second images for example. In otherembodiments the measuring device can be any instrument suitable formeasuring optical density—for example, a sensor or an opticaldensitometer. The measuring device 64 includes a processor 65 capable ofcommunicating with the printer controller 22 in order to provideinformation to the printer controller 22 regarding the optical ratiobetween a pair of images produced by the xerographic process.Alternatively the processor 65 may provide information only on theoptical densities of each image produced by the xerographic process andthe printer controller 22 may itself determine the optical ratio. If theoptical ratio is determined by the printer controller 22 (either itselfor by being informed by the processor 65) that the optical ratio betweentwo images is not at a desired level or not close enough to a desiredlevel then at step 56, further pairs of images are produced until thedesired optical ratio is reached. Therefore in this embodiment the stepof measuring optical densities and hence the optical ratio is automatic,i.e. there is no human input required. As previously described, in otherembodiments this step can be performed manually instead. In otherembodiments a combination of manual and automatic measuring can beemployed.

FIG. 14 shows an example of a third image 66 and a fourth image 68 whichcan be used to set a non-zero dot gain level using this calibrationprocess 48. In this case the third image comprises a repeating 2 on -2off horizontal line pattern and the fourth image 68 comprises ahorizontal 4 on -2 off pattern. In this case the light source level usedto produce the third and fourth images 66, 68 is the same again. Theaverage optical density of the third image 66 is (2+2 D₁)/4 and theaverage optical density of the fourth image 68 is (4+2 D₁)/6. Using thesame process as previously described, when the optical ratio is one, adot gain level of one (i.e. one pixel width) is achieved. Therefore thiscombination of patterns can be used to set a light source level toachieve a dot gain of one.

It can also be used to set a light source level at different dot gains.For example if the desired optical ratio at step 56 is set to three,i.e. the average optical density of the third image 66 is three times asmuch as the average optical density of the fourth image 68 then thelight source level used to produce the dot gain which provides thedesired optical ratio will be producing a dot gain of minus three. So ifthis particular dot gain is required then the third and fourth images66, 68 can also be used. It will be apparent that various differentgeometries can be used to provide various dot gain levels. It will alsobe apparent that the optical ratio does not need to be an integer: itcan be any number.

FIG. 15 shows a fifth image 70 and a sixth image 72. The fifth imagecomprises a 3 on -3 off horizontal repeating pattern produced at a lightsource level providing a first dot gain D_(x). The average opticaldensity of the fifth image 70 is therefore (3+2 D_(x))/6. The sixthimage comprises a 2 on -4 off horizontal repeating pattern producedusing a light source level which produces a dot gain D_(y) and thereforethe average optical density of the sixth image is (2+2 D_(y))/6. It isknown that D_(x)=0, i.e. the fifth image 70 is produced using a lightsource level which produces zero dot gain. Therefore if it is desired tofind the light source level which will produce a dot gain of 0.5 linewidth, i.e. D_(y)=0.5 then the desired optical ratio which is requiredto be found at step 56 of the calibration process is one i.e. when thefifth and sixth images, 70, 72 are as dark as each other then the lightsource level used to produce the sixth image 72 will be the light sourcelevel which produces a dot gain of 0.5 for that xerographic process. Itwill be appreciated that different patterns can be used and differentoptical ratios can be used to determine the light source level requiredto produce a particular dot gain in the second of a pair of images whenthe dot gain present in the first of the pair of images is known.

In other embodiments, the printer may print a series of first and secondimages and the optical ratio can be calculated (in a manner aspreviously described) for each pair of images. If a desired opticalratio is found, a desired light source level is established. If not,further pairs of images can be printed to arrive at the desired ratio.Alternatively, in other embodiments, the pair of images which providesan optical ratio closest to the desired optical ratio may be used toindicate a suitable light source level. This may be useful, for example,if it is not possible for a light source level to operate at a level toproduce the desired ratio and instead the most extreme operation modeclosest to the desired level is used.

It is also possible in some embodiments to calibrate for particulartypes of edges. For example when calibrating to provide a desired levelof dot gain at a diagonal edge or a circular edge the calibrationprocess may be applied to pairs of images having repeating patterns informs which more closely match those edges. For example when calibratingfor a diagonal edge, a pair of images comprising a diagonal shape orshapes may be used to calibrate the xerographic process.

Referring to FIG. 16, examples of images produced using a knownxerographic process are shown. Similarly FIG. 17 shows images producedusing a xerographic process including the dot gain compensation methodof FIG. 3.

Referring to FIGS. 18 a and 18 b, examples of a 2 on -2 off repeatinghorizontal pattern are shown having been produced by a standardxerographic process and by a xerographic process employing thecalibration process 48 of the present invention respectively.

Referring to FIGS. 19 a and 19 b, images comprising repeating 4 on -4off patterns produced by a standard xerographic process and axerographic process including the calibration process 48 of the presentinvention respectively are shown. Referring to FIGS. 20 a and 20 b,images comprising an “8 on -8 off” repeating pattern formed using astandard xerographic process and a xerographic process including thecalibration process 48 of the present invention are shown. From theseFigures it is apparent that the processes of the present inventionprovide clearer final images and images which more closely representimages intended to be produced by a xerographic process than standardxerographic processes.

1-20. (canceled)
 21. A method to decouple the control of dot-gain andoptical density in electro-photographic based printing by varying theexposure level for each dot in an image.
 22. The method of claim 21,wherein each image dot is classified as an edge dot or a non-edge dot,and wherein non-edge dots are attributed an exposure level that controlsthe optical density, and wherein edge dots are attributed an exposurelevel to control dot gain.
 23. The method of claim 22, wherein edge-dotsare classified into stable edge-dots and unstable edge-dots that mightfail to print and the method comprises using an exposure level forunstable edge dots so that the corresponding dot is printed reliably.24. The method of claim 21, wherein the exposure source acts differentlyby (i) operating for a different period of time, (ii) operating indifferent bursts, (iii) operating at a different intensity, (iv)scanning exposure across the surface at a different rate, or (v)exposing the surface by a different amount when forming a dot in anyother suitable way, or (vi) any combination of (i) to (v).
 25. Themethod of claim 21, wherein an input digital image is digitallyprocessed to generate a second digital image comprising of exposure tagsfor each dot of a corresponding latent image; wherein each exposure tagcorresponds to a particular exposure level for a dot in the latentimage.
 26. The method of claim 25, wherein the set of exposure tagsincludes at least one tag for non-edge dots, at least one tag for stableedge dots and at least one tag for non-stable edge dots.
 27. The methodof claim 24, wherein the exposure tag corresponding to each pixel in theinput image is determined by analysis of pixels in its localneighbourhood.
 28. The method of claim 25, wherein the exposure levelcorresponding to non-edge tags is set such that printed solid patcheshave a desired optical density, and wherein the exposure levelscorresponding to edge tags are set such that non-solid printed patternshave a desired dot-gain.
 29. The method of claim 28, wherein the desiredlevel of dot gain is substantially zero.
 30. The method of claim 26,wherein the exposure levels of unstable edge tags are set to the minimumvalues possible that still ensure that the corresponding dots areprinted reliably.
 31. The method of claim 23, wherein the exposure levelof unstable edge dots is higher than the exposure level for stable edgedots.
 32. The method of claim 21 comprising converting a digital image;comprising pixels into a latent image comprising dots using light from acontrollable light source arranged to strike a photoconductive surfaceand change charge distribution on the surface to form the dots makingthe latent image on the surface, the digital and latent images eachhaving an edge and comprising an edge pixel or edge dot respectively,which is at or near the edge, and a non-edge pixel or non-edge dotrespectively, which is not at or near the edge, wherein the methodcomprises the step of identifying whether or not a dot to be formed isan edge dot and using a different light source level incident at thephotoconductive surface when forming the edge dot compared to whenforming the non-edge dot such that charge distribution is changed to adifferent extent when forming the edge dot compared to when forming thenon-edge dot.
 33. The method of claim 32, wherein each pixel of thedigital image has an associated instruction indicating a default lightsource level which should be used when forming its corresponding dot inthe latent image, the method comprising forming the edge dot using alight source level different to the default light source level.
 34. Amethod of controlling a light source used in a xerographic process, theprocess comprising converting a digital image comprising pixels into aphysical image comprising corresponding dots, the method arranged toachieve a desired light source level such that charge distribution on aphotoconductive surface is changed to a desired extent and achieve adesired level of dot gain in physical images produced by the process,the light source being operable in a plurality of modes to producediffering levels of dot gain; an optical ratio between twoxerographically produced physical images being defined as a ratio ofmean average optical densities of each image; the method comprisingusing the xerographic process to produce a first physical image having afirst mean average optical density and a first attribute whichinfluences the mean average optical density of the image for a givenlevel of dot gain and a second physical image having a second averageoptical density and a second attribute which influences the averageoptical density of the image for a given level of dot gain, the firstand second physical images with their associated first and secondattributes being such that at a particular optical ratio between thefirst and second physical images, the level of dot gain in the secondphysical image will be at the desired level, the method comprisingadjusting the light source level to produce first and second physicalimages until they substantially provide the desired optical ratiobetween the xerographically produced physical images, and therebyestablishing the desired light source level.
 35. The method of claim 34,wherein each physical image comprises edge dots at an edge of the imageand non-edge dots not at the edge and the attribute comprises the ratioof area covered by edge pixels to area covered by all pixels for eachimage.
 36. The method of claim 34, wherein the light source level usedto produce the first and second physical images is the same and theoptical ratio is one, or the light source level used to produce thefirst and second physical images is the same and the optical ratio isnot one, or the light source level used to produce the first and secondphysical images is not the same and the level of dot gain in the firstphysical image is known.
 37. The method of claim 34, wherein the step ofadjusting the light source level to produce first and second physicalimages which substantially provide the desired optical ratio comprisesthe step of measuring the first mean average optical density, producingone or more further physical images, measuring the average opticaldensity for each of the further physical images to find a physical imagewhich substantially has the desired second mean average optical density.38. The method of claim 37, wherein the measuring step comprises anautomatic measuring step without requiring human input.
 39. Axerographic machine, such as a printer or a photocopier arranged tocarry out the method of claim
 21. 40. Software which when run on aprocessor, e.g. of a xerographic machine is arranged to cause the methodof claim 21 to be carried out.